Common magnetic-field sensors for converting magnetic field to electric field signals include reluctance coils and Hall-effect devices. Reluctance coils generate an electric voltage proportional in magnitude to the time rate of change of magnetic flux coupling within the coils. To obtain an accurate measurement, a highly precise, low-noise, low-drift electronic integrator may be required to integrate the voltage signal induced across the coil. However, the lower the signal frequency (i.e., the lower the flux change rate), the longer the integration time is required, and below a certain signal frequency, the voltage signal disappears into the noise. Although Hall-effect devices do not suffer from these problems, they have limited sensitivities (i.e., 5–50 μV/Oe) and are always hampered by a noise-induced bandwidth limitation to about 30 kHz. In addition, they require a highly stable constant-current source to establish an accurate Hall voltage output.
Magnetoelectric devices have received continuous attention due to their distinct advantage of providing a relatively simple, cost-effective and reliable means for direct-conversion of magnetic fields to electric fields and vice versa. Magnetoelectric effect is defined as a variation of dielectric polarization in a material when subjected to an applied magnetic field, or an induced magnetization in response to an external electric field. In recent years, several bulk and laminate magnetoelectric two-phase materials have been created to overcome the drawbacks of low operational temperatures and low magnetoelectric effect in single-phase materials.
Bulk materials may be represented by sintered 0–3 composites of magnetostrictive ferrite (i.e., a metal-iron-oxide ceramic) particles [e.g., cobalt ferrite (CFO or CoFe2O4), nickel ferrite (NFO or NiFe2O4), copper ferrite (CuFe2O4), manganese chromium ferrite (MnFe2Cr0.2O4), cobalt zinc ferrite (CZFO), nickel zinc ferrite (NZFO), lithium zinc ferrite (LZFO), etc.] dispersed in a piezoelectric ceramic matrix [e.g., barium titanate (BaTiO3), lead zirconate titanate (PZT), etc.] (J. van den Boomgaard and R. A. J. Born, “A Sintered Magnetoelectric Composite Material BaTiO3—Ni(Co,Mn)Fe2O4”, J. Mater. Sci., vol. 13, pp. 1538–1548, 1978). While these sintered bulk materials generally show a higher magnetoelectric voltage coefficient (i.e., αE˜0.13 V/cm·Oe, where αE=dE/dH is the ratio of the change in electric field strength to the change in magnetic field strength) than single-phase materials (i.e., ˜0.02 V/cm·Oe for Cr2O3), they still have some crucial problems in reproducibility and reliability impeding their commercial viability. These problems include: 1) difficulties in machining and fabricating devices due to the brittleness of the materials; 2) difficulties in controlling the connectivity of the constituent phases; 3) chemical reaction between phases during high-temperature sintering; 4) dielectric breakdown through the low electrically resistant magnetostrictive phase during poling of the piezoelectric phase under a high electric poling field to induce an electric polarization; and 5) weak mechanical coupling between phases owing to processing-induced mechanical defects (i.e., pores, cracks, etc.).
Laminate materials may include bilayer, sandwich and multilayer structures of either magnetostrictive metal plates/disks [e.g., terbium-dysprosium-iron alloy (Terfenol-D), iron (Fe), cobalt (Co), nickel (Ni), etc.] or magnetostrictive ferrite plates/disks (e.g., CFO, NFO, CZFO, NZFO, LZFO, etc.) and piezoelectric ceramic plates/disks [e.g., BaTiO3, PZT, lead magnesium niobate-lead titanate (PMN-PT), lead zirconate niobate-lead titanate (PZN-PT), etc.] (W. N. Podney, “Composite Structured Piezomagnetometer”, U.S. Pat. No. 5,675,252, 7 Oct. 1997). Among these laminate structures, the ones incorporating Terfenol-D, a magnetostrictive rare-earth-based alloy of terbium (Tb), dysprosium (Dy) and iron (Fe), exhibit the greatest magnetoelectric voltage coefficient αE. This may be due to the giant magnetostrictive strain (i.e., ˜1200 ppm) produced by Terfenol-D in comparison with other magnetostrictive materials (i.e., only on an order of 10 ppm). An effective mechanical coupling between the magnetostrictive and piezoelectric phases related to simple structure and simple fabrication technique (i.e., bonding all well-prepared constituent layers together) plays another key factor to produce the high magnetoelectric voltage coefficient αE.
As the development of magnetoelectric materials so far relies on the use of metallic or ceramic magnetostrictive materials and ceramic piezoelectric materials as their constituent phases, this leads to three significant problems in the resulting magnetoelectric devices. The first is the limitation of the operational frequency to a few kilohertz due to the presence of eddy-current losses in the low electrically resistant metallic magnetostrictive phase (i.e., electrical resistivity ˜0.6 μΩ·m for Terfenol-D). The second is difficulties in machining and fabricating devices owing to the mechanical brittleness of the ceramic and some metallic (i.e., Terfenol-D, etc.) magnetostrictive phases as well as of the ceramic piezoelectric phase. The third is difficulties in tailoring and optimizing the properties (i.e., magnetoelectric voltage coefficient αE, operational frequency range, etc.) of the devices due to the limitation of the types of the constituent materials. Particularly, the problem arisen from eddy-current losses may have reduced the commercial and practical values of currently available magnetoelectric devices, since their operational frequencies (i.e., a few kilohertz only) are even lower than those of traditional Hall-effect devices (i.e., ˜30 kHz). This problem, together with that caused by limitation of materials' types, may have restricted the existing magnetoelectric devices to be only used as a low-frequency sensor or a low-frequency transducer.